In recent years, in a communication device such as a mobile terminal, use is made of an acoustic wave element for a duplexer for filtering signals which are transmitted to and received from an antenna. Such an acoustic wave element is configured by a piezoelectric substrate and an excitation electrode formed on a major surface of the piezoelectric substrate. The acoustic wave element utilizes the characteristic feature that an electrical signal and a surface acoustic wave can be converted to each other by the relationship between the excitation electrode and the piezoelectric substrate.
A duplexer uses a plurality of such acoustic wave elements to configure for example a receiving filter (Rx filter) or transmission filter (see Japanese Patent Publication No. 2007-214902A etc.). In the duplexer, the plurality of acoustic wave elements are used to set a passband of the reception band or transmission band.